The present invention relates to an optical transmitting and receiving apparatus of a heterodyne detecting system applicable to a coherent optical transmission which is expected to be applied to a long-distance large capacity communication, and more particularly to an improvement of a circuit portion for positively controlling the polarization state of a signal light.
In coherent optical communication, a heterodyne detecting system is used to detect a signal light received through a light transmission path, and to also detect a local oscillation light. However, in such a system, the polarization state of a signal varies during its period of transmission along an optical fiber. Thus, when a signal light is mixed with a local oscillation light, there is the problem that the detecting efficiency is deteriorated. To solve this problem, a method is required for positively controlling the polarization states of a signal light and a local oscillation light. FIG. 1 shows a circuit structure of a prior art heterodyne detecting receiving apparatus used to achieve this positive control.
In the drawing, the signal light transmitted through an optical fiber is inputted to mixing circuit 2 through polarization apparatus 1 and is then mixed with local oscillation light outputted from optical local oscillating circuit 3. The local oscillating light is assumed to be in a polarized state in which the principal axis angle is slanted, for example, by 45 degrees and the elliptic ratio is 1. One output signal from mixing circuit 2 is subjected to heterodyne detection by optical receiver 4 and is converted to an intermediate frequency signal. This intermediate frequency signal is demodulated by demodulator 5 and outputted therefrom.
The other output signal from mixing circuit 2 is used to monitor the polarization state of the signal light. Namely, the optical signal for monitoring is first divided into two portions by half mirror 6 and the transparent light is separated by polarization splitter 7 into two polarization components which intersect at right angles. The optical signal is thereafter converted into an electrical signal by optical receivers 8 and 9. The light reflected from half mirror 6 is subjected to a conversion of its polarization state by .lambda./4 plate 10 (for example, a linearly polarized light is converted to a circularly polarized light or a circularly polarized light is converted into a linearly polarized light) and subsequently the reflected light is separated into two orthogonal polarization components by polarization splitter 11, whose axis is rotated 45 degrees from that of polarization splitter 7 and is thereafter converted into an electrical signal by optical receivers 12 and 13. The output signals from optical receivers 8 and 9 are inputted to differential amplifier 14 and the output signals from optical receivers 12 and 13 are inputted to the other differential amplifier 15. Differential amplifier signals A and B obtained from differential amplifiers 14 and 15 are used as monitor signals by polarization operation apparatus 1 to control the polarization state of the signal light. For example, the principal axis angle of the polarization state of the signal light is controlled by making signal A equal to 0 and the elliptic ratio is controlled by making signal B equal to 0.
In the prior art heterodyne detecting receiver described above, a part of the signal light is divided into two portions by half mirror 6 and polarization splitters 7 and 11, and the polarization state of the signal light is monitored by four optical receivers 8, 9, 12 and 13, thus controlling the polarization state of the signal light.
Accordingly, the structure of the optical system becomes complicated as half mirror 6, .lambda./4 plate 10, the two polarization splitters 7 and 11, and the four optical receivers 8, 9, 12 and 13 must be used for merely monitoring the polarization state of signal light, even when optuical receivers are required, resulting in the problem that the apparatus must be large. A part of the signal light is divided and used only for monitoring. Thus, the receiving sensitivity is deteriorated by an amount corresponding to the use of such a monitoring system. Further, there is a problem that the prior art apparatus cannot form a DBOR (Dual Balanced Optical Signal Receiver, refer to Japanese Patent Kokai Nos. 63-19928 and 63-1124, U.S. application Ser. No. 064058/87, EPC Application No. 87108787, Canadian Application No. 539613/87) which makes it possible to perform a high-sensitivity receiving operation by using the signal component effectively supressing the noise component.